The quantum yield for CO2 uptake in C3 and C4 grasses

The quantum yield for CO₂ uptake was measured in C₃ and C₄ monocot species from several different grassland habitats. When the quantum yield was measured in the presence of 21% O₂ and 340 cm³ m^-3 CO₂, values were very similar in C₃ monocots, C₃ dicots, and C₄ monocots (0.045–0.056 mole CO₂ · mole^-1 quanta absorbed). In the presence of 2% O₂ and 800 cm³ m^-3 CO₂, enhancements of the quantum yield values occurred for the C₃ plants (both monocots and dicots), but not for C₄ monocots. A dependence of the quantum yield on leaf temperature was observed in the C₃ grass, Agropyron smithii, but not in the C₄ grass, Bouteloua gracilis, in 21% O₂ and 340 cm³ m^-3 CO₂. At leaf temperatures between 22–25°C the quantum yield values were approximately equal in the two species.     

The C4 pathway: an efficient CO2 pump

The C₄ pathway is a complex combination of both biochemical and morphological specialisation, which provides an elevation of the CO₂ concentration at the site of Rubisco. We review the key parameters necessary to make the C₄ pathway function efficiently, focussing on the diffusion of CO₂ out of the bundle sheath compartment. Measurements of cell wall thickness show that the thickness of bundle sheath cell walls in C₄ species is similar to cell wall thickness of C₃ mesophyll cells. Furthermore, NAD-ME type C₄ species, which do not have suberin in their bundle sheath cell walls, do not appear to compensate for this with thicker bundle sheath cell walls. Uncertainties in the CO₂ diffusion properties of membranes, such as the plasmalemma, choroplast and mitochondrial membranes make it difficult to estimate bundle sheath diffusion resistance from anatomical measurements, but the cytosol itself may account for more than half of the final calculated resistance value for CO₂ leakage. We conclude that the location of the site of decarboxylation, its distance from the mesophyll interface and the physical arrangement of chloroplasts and mitochondria in the bundle sheath cell are as important to the efficiency of the process as the properties of the bundle sheath cell wall. Using a mathemathical model of C₄ photosynthesis, we also examine the relationship between bundle sheath resistance to CO₂ diffusion and the biochemical capacity of the C₄ photosynthetic pathway and conclude that bundle sheath resistance to CO₂ diffusion must vary with biochemical capacity if the efficiency of the C₄ pump is to be maintained. Finally, we construct a mathematical model of single cell C₄ photosynthesis in a C₃ mesophyll cell and examine the theoretical efficiency of such a C₄ photosynthetic CO₂ pump. This revised version was published online in August 2006 with corrections to the Cover Date.     

Evolution of C4 plants: a new hypothesis for an interaction of CO2 and water relations mediated by plant hydraulics

C(4) photosynthesis has evolved more than 60 times as a carbon-concentrating mechanism to augment the ancestral C(3) photosynthetic pathway. The rate and the efficiency of photosynthesis are greater in the C(4) than C(3) type under atmospheric CO(2) depletion, high light and temperature, suggesting these factors as important selective agents. This hypothesis is consistent with comparative analyses of grasses, which indicate repeated evolutionary transitions from shaded forest to open habitats. However, such environmental transitions also impact strongly on plant-water relations. We hypothesize that excessive demand for water transport associated with low CO(2), high light and temperature would have selected for C(4) photosynthesis not only to increase the efficiency and rate of photosynthesis, but also as a water-conserving mechanism. Our proposal is supported by evidence from the literature and physiological models. The C(4) pathway allows high rates of photosynthesis at low stomatal conductance, even given low atmospheric CO(2). The resultant decrease in transpiration protects the hydraulic system, allowing stomata to remain open and photosynthesis to be sustained for longer under drying atmospheric and soil conditions. The evolution of C(4) photosynthesis therefore simultaneously improved plant carbon and water relations, conferring strong benefits as atmospheric CO(2) declined and ecological demand for water rose.     

Incorporation of 14CO2 into C4 acids by leaves of C3-C4 intermediate and C3 species of Moricandia and Panicum at the CO2 compensation concentration

Comparative ¹⁴CO₂ pulse-¹²CO₂ chase studies performed at CO₂ compensation ()-versus air-concentrations of CO₂ demonstrated a four-to eightfold increase in assimilation of ¹⁴CO₂ into the C₄ acids malate and aspartate by leaves of the C₃-C₄ intermediate species Panicum milioides Nees ex Trin., P. decipiens Nees ex Trin., Moricandia arvensis (L.) DC., and M. spinosa Pomel at . Specifically, the distribution of ¹⁴C in malate and aspartate following a 10-s pulse with ¹⁴CO₂ increases from 2% to 17% (P. milioides) and 4% to 16% (M. arvensis) when leaves are illuminated at the CO₂ compensation concentration (20 l CO₂/l, 21% O₂) versus air (340 l CO₂/l, 21% O₂). Chasing recently incorporated ¹⁴C for up to 5 min with ¹²CO₂ failed to show any substantial turnover of label in the C₄ acids or in carbon-4 of malate. The C₄-acid labeling patterns of leaves of the closely related C₃ species, P. laxum Sw. and M. moricandioides (Boiss.) Heywood, were found to be relatively unresponsive to changes in pCO₂ from air to . These data demonstrate that the C₃-C₄ intermediate species of Panicum and Moricandia possess an inherently greater capacity for CO₂ assimilation via phosphoenolpyruvate (PEP) carboxylase (EC 4.1.1.31) at the CO₂ compensation concentration than closely related C₃ species. However, even at , CO₂ fixation by PEP carboxylase is minor compared to that via ribulosebisphosphate carboxylase (EC 4.1.1.39) and the C₃ cycle, and it is, therefore, unlikely to contribute in a major way to the mechanism(s) facilitating reduced photorespiration in the C₃-C₄ intermediate species of Panicum and Moricandia.Abbreviations Rubisco ribulose-1,5-bisphosphate carboxylase/oxygenase - PEP phosphoenolpyruvate - CO₂ compensation concentration - 3PGA 3-phosphoglycerate - SuP sugar monophosphates - SuP₂ sugar bisphosphates Published as Paper No. 8249, Journal Series, Nebraska Agricultural Research Division     

Effects of irradiance and temperature on photosynthesis in C3, C4 and C3/C4 Panicum species

Species in the Laxa and Grandia groups of the genus Panicum are adapted to low, wet areas of tropical and subtropical America. Panicum milioides is a species with C₃ photosynthesis and low apparent photorespiration and has been classified as a C₃/C₄ intermediate. Other species in the Laxa group are C₃ with normal photorespiration. Panicum prionitis is a C₄ species in the Grandia group. Since P. milioides has some leaf characteristics intermediate to C₃ and C₄ species, its photosynthetic response to irradiance and temperature was compared to the closely related C₃ species, P. laxum and P. boliviense and to P. prionitis. The response of apparent photosynthesis to irradiance and temperature was similar to that of P. laxum and P. boliviense, with saturation at a photosynthetic photo flux density of about 1 mmol m^-2 s^-1 at 30°C and temperature optimum near 30°C. In contrast, P. prionitis showed no light saturation up to 2 mmol m^-2 s^-1 and an optimum temperature near 40°C. P. milioides exhibited low CO₂ loss into CO₂-free air in the light and this loss was nearly insensitive to temperature. Loss of CO₂ in the light in the C₃ species, P. laxum and P. boliviense, was several-fold higher than in P. milioides and increased 2- to 5-fold with increases in temperature from 10 to 40°C. The level of dark respiration and its response to temperature were similar in all four Panicum species examined. It is concluded that the low apparent photorespiration in P. milioides does not influence its response of apparent photosynthesis to irradiance and temperature in comparison to closely related C₃ Panicum species.Abbreviations AP apparent photosynthesis - I CO₂ compensation point - gl leaf conductance; gm, mesophyll conductance - PPFD photosynthetic photon flux density - PR apparent photorespiration rate - RuBPC sibulose bisphosphate carboxylase     

Identification of C4 responsive genes in the facultative C4 plant Hydrilla verticillata

The aquatic monocot Hydrilla verticillata (L.f.) Royle is a well-documented facultative C₄ NADP-malic enzyme species in which the C₄ and Calvin cycles operate in the same cell with the specific carboxylases confined to the cytosol and chloroplast, respectively. Several key components had already been characterized at the molecular level, thus the purpose of this study was to begin to identify other, less obvious, elements that may be necessary for a functional single-cell C₄ system. Using differential display, mRNA populations from C₃ and C₄ H. verticillata leaves were screened and expression profiles compared. From this study, 65 clones were isolated and subjected to a customized macroarray analysis; 25 clones were found to be upregulated in C₄ leaves. Northern and semi-quantitative RT-PCR analyses were used for confirmation. From these screenings, 13 C₄ upregulated genes were identified. Among these one encoded a previously recognized C₄ phosphoenolpyruvate carboxylase, and two encoded distinct pyruvate orthophosphate dikinase isoforms, new findings for H. verticillata. Genes that encode a transporter, an aminotransferase and two chaperonins were also upregulated. Twelve false positives, mostly housekeeping genes, were determined from the Northern/semi-quantitative RT-PCR analyses. Sequence data obtained in this study are listed in the dbEST database (DV216698 to DV216767). As a single-cell C₄ system that lacks Kranz anatomy, a better understanding of how H. verticillata operates may facilitate the design of a transgenic C₄ system in a C₃ crop species.Srinath K. Rao and Hiroshi Fukayama contributed equally to this study.     

Responses of C4 grasses to atmospheric CO2 enrichment

Summary The growth and photosynethetic responses to atmospheric CO₂ enrichment of 4 species of C₄ grasses grown at two levels of irradiance were studied. We sought to determine whether CO₂ enrichment would yield proportionally greater growth enhancement in the C₄ grasses when they were grown at low irradiance than when grown at high irradiance. The species studied were Echinochloa crusgalli, Digitaria sanguinalis, Eleusine indica, and Setaria faberi. Plants were grown in controlled environment chambers at 350, 675 and 1,000 l 1^-1 CO₂ and 1,000 or 150 mol m^-2 s^-1 photosynthetic photon flux density (PPFD). An increase in CO₂ concentration and PPFD significantly affected net photosynthesis and total biomass production of all plants. Plants grown at low PPFD had significantly lower rates of photosynthesis, produced less biomass, and had reduced responses to increases in CO₂. Plants grown in CO₂-enriched atmosphere had lower photosynthetic capacity relative to the low CO₂ grown plants when exposed to lower CO₂ concentration at the time of measurement, but had greater rate of photosynthesis when exposed to increasing PPFD. The light level under which the plants were growing did not influence the CO₂ compensation point for photosynthesis.     

Co-function of C3-and C4-photosynthetic pathways in C3, C4 and C3-C4 intermediate Flaveria species

The potential for C₄ photosynthesis was investigated in five C₃-C₄ intermediate species, one C₃ species, and one C₄ species in the genus Flaveria, using ¹⁴CO₂ pulse-¹²CO₂ chase techniques and quantum-yield measurements. All five intermediate species were capable of incorporating ¹⁴CO₂ into the C₄ acids malate and aspartate, following an 8-s pulse. The proportion of ¹⁴C label in these C₄ products ranged from 50–55% to 20–26% in the C₃-C₄ intermediates F. floridana Johnston and F. linearis Lag. respectively. All of the intermediate species incorporated as much, or more, ¹⁴CO₂ into aspartate as into malate. Generally, about 5–15% of the initial label in these species appeared as other organic acids. There was variation in the capacity for C₄ photosynthesis among the intermediate species based on the apparent rate of conversion of ¹⁴C label from the C₄ cycle to the C₃ cycle. In intermediate species such as F. pubescens Rydb., F. ramosissima Klatt., and F. floridana we observed a substantial decrease in label of C₄-cycle products and an increase in percentage label in C₃-cycle products during chase periods with ¹²CO₂, although the rate of change was slower than in the C₄ species, F. palmeri. In these C₃-C₄ intermediates both sucrose and fumarate were predominant products after a 20-min chase period. In the C₃-C₄ intermediates, F. anomala Robinson and f. linearis we observed no significant decrease in the label of C₄-cycle products during a 3-min chase period and a slow turnover during a 20-min chase, indicating a lower level of functional integration between the C₄ and C₃ cycles in these species, relative to the other intermediates. Although F. cronquistii Powell was previously identified as a C₃ species, 7–18% of the initial label was in malate+aspartate. However, only 40–50% of this label was in the C-4 position, indicating C₄-acid formation as secondary products of photosynthesis in F. cronquistii. In 21% O₂, the absorbed quantum yields for CO₂ uptake (in mol CO₂·[mol quanta]^-1) averaged 0.053 in F. cronquistii (C₃), 0.051 in F. trinervia (Spreng.) Mohr (C₄), 0.052 in F. ramosissima (C₃-C₄), 0.051 in F. anomala (C₃-C₄), 0.050 in F. linearis (C₃-C₄), 0.046 in F. floridana (C₃-C₄), and 0.044 in F. pubescens (C₃-C₄). In 2% O₂ an enhancement of the quantum yield was observed in all of the C₃-C₄ intermediate species, ranging from 21% in F. ramosissima to 43% in F. pubescens. In all intermediates the quantum yields in 2% O₂ were intermediate in value to the C₃ and C₄ species, indicating a co-function of the C₃ and C₄ cycles in CO₂ assimilation. The low quantum-yield values for F. pubescens and F. floridana in 21% O₂ presumably reflect an ineffcient transfer of carbon from the C₄ to the C₃ cycle. The response of the quantum yield to four increasing O₂ concentrations (2–35%) showed lower levels of O₂ inhibition in the C₃-C₄ intermediate F. ramosissima, relative to the C₃ species. This indicates that the co-function of the C₃ and C₄ cycles in this intermediate species leads to an increased CO₂ concentration at the site of ribulose-1,5-bisphosphate carboxylase/oxygenase and a concomitant decrease in the competitive inhibition by O₂.Abbreviations PEP phosphoenolpyruvate - PGA 3-phosphoglycerate - RuBP ribulose-1,5-bisphosphate     

Le vie fotosintetiche C3, C4 e CAM nel contesto ecologico

Abstract

Ecological aspects of C₃, C₄ and CAM photosynthetic pathways. - Three different photosynthetic CO₂ fixation pathways are known to occur in higher plants. However all three pathways ultimately depend on the Calvin-Benson cycle for carbon reduction. The oxygenase activity of RuBP carboxilase is responsible for photorespiratory CO₂ release. Both C₄ and CAM pathways behave as a CO₂ concentrating mechanism which prevent photorespiration. The CO₂-concentrating mechanism in C₄ plants is based on intracellular symplastic transport of C₄ dicarboxylic acids from mesophyll-cells to the adjacent bundle-sheath cells. On the contrary in CAM plants the CO₂-concentrating mechanism is based on the intracellular transport of malic acid into and out of the vacuole.

The C₄ photosynthetic pathway as compared to the C₃ pathway permits higher rates of CO₂ fixation in high light and high temperature environments at low costs in terms of water loss, given the stability of the photosynthetic apparatus under such conditions.

CAM is interpreted as an adaptation to arid environments because it enables carbon assimilation to take place at very low water costs during the night when the evaporative demand is low. Nevertheless many aquatic species of Isoetes and some relatives are CAM, suggesting the adaptive role of CAM to environments which become depleted in CO₂.

The photosynthetic carbon fixation pathway certainly contributes to the ecological success of plants in different environments. However the distribution of plants may also reflect their biological history. On the other hand plants with different photosynthetic pathways coexist in many communities and tend to share resources in time. In any case some generalizations are possible: C₄ plants enjoy an ecological advantage in hot, moist, high light regions while the majority of species in desert environments are C₃; CAM plants are more frequent in semiarid regions with seasonal rainfall, coastal fog deserts, and in epiphytic habitats in tropical rain forests.     

Structural characterization of photosynthetic cells in an amphibious sedge, Eleocharis vivipara, in relation to C3 and C4 metabolism

Eleocharis vivipara Link is a unique amphibious leafless sedge. The terrestrial form has Kranz anatomy and the biochemical traits of C₄ plants while the submerged form develops structural and biochemical traits similar to those of C₃ plants. The structural features of the culms, which are the photosynthetic organs, of the two forms were examined and compared. The culms of the terrestrial form have mesophyll cells and three bundle sheaths which consist of three kinds of cell, namely, the innermost Kranz cells that contain large numbers of organelles, the middle mestome sheath cells that lack chloroplasts, and the outermost parenchyma sheath cells that contain chloroplasts. The culms of the submerged form had a tendency towards reduction in numbers and size of Kranz cells and vascular bundles, as compared to the terrestrial form, and they had spherical mesophyll cells that were tightly packed without intercellular spaces inside the epidermis. The submerged form had a higher ratio of cross-sectional area of mesophyll cells plus parenchyma sheath cells to that of Kranz cells than the terrestrial form. The difference was mainly due to a decrease in the number and the size of the Kranz cells and to a marked increase in the size of the mesophyll cells and the parenchyma sheath cells in the submerged form, as compared to the terrestrial form. The Kranz cells of the terrestrial form had basically the structural characteristics of plants of the NAD-malic enzyme type, with the exception of the intracellular location of organelles. The Kranz cells of the submerged form included only a few organelles, and the percentage of organelles partitioned to the Kranz cells was significantly smaller in the submerged form than in the terrestrial form. In addition, the size of chloroplasts of the Kranz cells was 60–70% of that of the terrestrial form. These structural differences between the two forms may be related to the functional differences in their mechanisms of photosynthesis.Abbreviations KC Kranz cell - MC mesophyll cell - PSC parenchyma sheath cell - NAD-ME NAD-malic enzyme - VB vascular bundle This study was supported by Grants-in-Aid from the Ministry of Agriculture, Forestry and Fisheries of Japan (Integrated Research Program for the Use of Biotechnological Procedures for Plant Breeding) and from the Science and Technology Agency of Japan (Enhancement of Center-of-Excellence, the Special Coordination Funds for Promoting Science and Technology).     

Using growth analysis to interpret competition between a C3 and a C4 annual under ambient and elevated CO2

Summary Detailed growth analysis in conjunction with information on leaf display and nitrogen uptake was used to interpret competition between Abutilon theophrasti, a C₃ annual, and Amaranthus retroflexus, a C₄ annual, under ambient (350 l l^-1) and two levels of elevated (500 and 700 l l^-1) CO₂. Plants were grown both individually and in competition with each other. Competition caused a reduction in growth in both species, but for different reasons. In Abutilon, decreases in leaf area ratio (LAR) were responsible, whereas decreased unit leaf rate (ULR) was involved in the case of Amaranthus. Mean canopy height was lower in Amaranthus than Abutilon which may explain the low ULR of Amaranthus in competition. The decrease in LAR of Abutilon was associated with an increase in root/shoot ratio implying that Abutilon was limited by competition for below ground resources. The root/shoot ratio of Amaranthus actually decreased with competition, and Amaranthus had a much higher rate of nitrogen uptake per unit of root than did Abutilon. These latter results suggest that Amaranthus was better able to compete for below ground resources than Abutilon. Although the growth of both species was reduced by competition, generally speaking, the growth of Amaranthus was reduced to a greater extent than that of Abutilon. Regression analysis suggests that the success of Abutilon in competition was due to its larger starting capital (seed size) which gave it an early advantage over Amaranthus. Elevated CO₂ had a positive effect upon biomass in Amaranthus, and to a lesser extent, Abutilon. These effects were limited to the early part of the experiment in the case of the individually grown plants, however. Only Amaranthus exhibited a significant increase in relative growth rate (RGR). In spite of the transitory effect of CO₂ upon size in individually grown plants, level of CO₂ did effect final biomass of competitively grown plants. Abutilon grown in competition with Amaranthus had a greater final biomass than Amaranthus at ambient CO₂ levels, but this difference disappeared to a large extent at elevated CO₂. The high RGR of Amaranthus at elevated CO₂ levels allowed it to overcome the difference in initial size between the two species.This study was supported by a grant from the US Department of Energy     

A mathematical model of C4 photosynthesis: The mechanism of concentrating CO2 in NADP-malic enzyme type species

A computer model comprising light reactions in PS II and PS I, electron-proton transport reactions in mesophyll and bundle sheath chloroplasts, all enzymatic reactions and most of the known regulatory functions of NADP-ME type C₄ photosynthesis has been developed as a system of differential budget equations for intermediate compounds. Rate-equations were designed on principles of multisubstrate-multiproduct enzyme kinetics. Some of the 275 constants needed (ΔG₀′ and K _m values) were available from literature and others (V _m) were estimated from reported rates and pool sizes. The model provided good simulations for rates of photosynthesis and pool sizes of intermediates under varying light, CO₂ and O₂. A basic novelty of the model is coupling of NADPH production via NADP-ME with ATP production and regulation of the C₃ cycle in bundle sheath chloroplasts. The functional range of the ATP/NADPH ratio in bundle sheath chloroplasts extends from 1.5 to 2.1, being energetically most efficient around 2. In the presence of such stoichiometry, the CO₂ concentrating function can be explained on the basis of two processes: (a) extra ATP consumption for starch and protein synthesis in bundle sheath leads to a faster NADPH and CO₂ import compared with CO₂ fixation in bundle sheath, and (b) the residual photorespiratory activity consumes RuBP by oxygenation, NADPH and ATP and causes the imported CO₂ to accumulate in bundle sheath cells. As a wider application, the model may be used for predicting results of genetic engineering of plants. This revised version was published online in June 2006 with corrections to the Cover Date.     

The roles of carbonic anhydrases in photosynthetic CO2 concentrating mechanisms

Cyanobacteria, algae, aquatic angiosperms and higher plants have all developed their own unique versions of photosynthetic CO₂ concentrating mechanisms (CCMs) to aid Rubisco in efficient CO₂ capture. An important aspect of all CCMs is the critical roles that the specialised location and function that various carbonic anhydrase enzymes play in the overall process, participating the interconversion of CO₂ and HCO₃ ⁻ species both inside and outside the cell. This review examines what we currently understand about the nature of the carbonic anhydrase enzymes, their localisation and roles in the various CCMs that have been studied in detail. This revised version was published online in August 2006 with corrections to the Cover Date.     

Photosynthesis,water use and growth of a C4 grass stand at high CO2 concentration

Leaf photosynthesis rate of the C₄ species Paspalum plicatulum Michx was virtually CO₂-saturated at normal atmospheric CO₂ concentration but transpiration decreased as CO₂ was increased above normal concentrations thereby increasing transpiration efficiency. To test whether this leaf response led growth to be CO₂-sensitive when water supply was restricted, plants were grown in sealed pots of soil as miniature swards. Water was supplied either daily to maintain a constant water table, or at three growth restricting levels on a 5-day drying cycle. Plants were either in a cabinet with normal air (340 mol (CO₂) mol^-1 (air)) or with 250 mol mol^-1 enrichment. Harvesting was by several cycles of defoliation.With abundant water supply high CO₂ concentration did not cause increased growth, but it did not cause an increase in growth over a wide range of growth-limiting water supplies either. Only when water supply was less than 30–50% of the amount used by the stand with a water-table was there evidence that dry weight growth was enhanced by high CO₂. In addition, with successive regrowth, the enhancing effect under a regime of minimal water allocations, became attenuated. Examination of leaf gas exchange, growth and water use data showed that in the long term stomatal conductance responses were of little significance in matching plant water use to low water allocation; regulation of leaf area was the mechanism through which consumption matched supply. Since high CO₂ effects operate principally via stomatal conductance in C₄ species, we postulate that for this species higher CO₂ concentrations expected globally in future will not have much effect on long term growth.     

The occurrence of both C3 and C4 photosynthetic characteristics in a single Zea mays plant

The activities of the carboxylating enzymes ribulose-1,5-biphosphate (RuBP) carboxylase and phosphoenolpyruvate (PEP) carboxylase in leaves of three-week old Zea mays plants grown under phytotron conditions were found to vary according to leaf position. In the lower leaves the activity of PEP carboxylase was lower than that of RuBP carboxylase, while the upper leaves exhibited high levels of PEP carboxylase. Carbon dioxide compensation points and net photosynthetic rates also differed in the lower and upper leaves. Differences in the fine structure of the lowermost and uppermost leaves are shown. The existence of both the C₃ and C₄ photosynthetic pathways in the same plant, in this and other species, is discussed.Abbreviations PEP phosphoenolpyruvate - RuBP ribulose-1,5-biphosphate     

Long-term photosynthetic response in single leaves of A C3 and C4 salt marsh species grown at elevated atmospheric CO2 in situ

Summary Mono-specific communities of the C₃ sedge, Scirpus olneyi and the C₄ grass, Spartina patens, were exposed to normal ambient or elevated CO₂, (ca. 680 l l^–1) throughout the 1987 and 1988 growing seasons in open-top field chambers located on a tidal marsh. Single stems of C₃ plants grown in ambient or elevated CO₂ showed an increased photosynthetic rate when tested at elevated CO₂ for both seasons. This increase in photosynthetic response in the C₃ species was maintained throughout the 1987 and 1988 growing season. The stimulation of photosynthesis with elevated CO₂ appeared to increase as temperature increased and decreased as photosynthetic photon flux (PPF) increased. Analysis of the photosynthetic response of the C₃ species during the 1988 season indicated that significant differences in light-saturated photosynthetic rate between ambient and elevated CO₂ conditions continued until October. In contrast to the C₃ sedge, the C₄ grass showed no significant photosynthetic increase to elevated CO₂ except at the beginning of the 1988 season.     

Biochemistry of C3-photosynthesis in high CO2

The short-term responses of C₃ photosynthesis to high CO₂ are described first. Regulation of photosynthesis in the short term is determined by interaction among the capacities of light harvesting, electron transport, ribulose-1, 5-bisphosphate carboxylase (Rubisco) and orthophosphate (Pi) regeneration during starch and sucrose synthesis. Photosynthesis under high CO₂ conditions is limited by either electron transport or Pi regeneration capacities, and Rubisco is deactivated to maintain a balance between each step in the photosynthetic pathway. Subsequently, the long-term effects on, photosynthesis are discussed. Long-term CO₂ enhancement leads to carbohydrate accumulation. Accumulation of carbohydrates is not associated with a Pi-regeneration limitation on photosynthesis, and this limitation is apparently removed during long-term exposure to high CO₂. Enhanced CO₂ does not affect Rubisco content and electron transport capacity for a given leaf-nitrogen content. In addition, the deactivated Rubisco immediately after exposure to high CO₂ does not recover during the subsequent prolonged exposure. Such evidence may indicate that plants do not necessarily have an ideal acclimation response to high CO₂ at the biochemical level.     

Effects of low and elevated CO2 on C3 and C4 annuals

Abutilon theophrasti (C₃) and Amaranthus retroflexus (C₄), were grown from seed at four partial pressures of CO₂: 15 Pa (below Pleistocene minimum), 27 Pa (pre-industrial), 35 Pa (current), and 70 Pa (future) in the Duke Phytotron under high light, high nutrient, and wellwatered conditions to evaluate their photosynthetic response to historic and future levels of CO₂. Net photosynthesis at growth CO₂ partial pressures increased with increasing CO₂ for C₃ plants, but not C₄ plants. Net photosynthesis of Abutilon at 15 Pa CO₂ was 70% less than that of plants grown at 35 Pa CO₂, due to greater stomatal and biochemical limitations at 15 Pa CO₂. Relative stomatal limitation (RSL) of Abutilon at 15 Pa CO₂ was nearly 3 times greater than at 35 Pa CO₂. A photosynthesis model was used to estimate ribulose-1,5-bisphosphate carboxylase (rubisco) activity (Vc_max), electron transport mediated RuBP regeneration capacity (J _max), and phosphate regeneration capacity (PiRC) in Abutilon from net photosynthesis versus intercellular CO₂ (A–C _i) curves. All three component processes decreased by approximately 25% in Abutilon grown at 15 Pa compared with 35 Pa CO₂. Abutilon grown at 15 Pa CO₂ had significant reductions in total rubisco activity (25%), rubisco content (30%), activation state (29%), chlorophyll content (39%), N content (32%), and starch content (68%) compared with plants grown at 35 Pa CO₂. Greater allocation to rubisco relative to light reaction components and concomitant decreases in J _max and PiRC suggest co-regulation of biochemical processes occurred in Abutilon grown at 15 Pa CO₂. There were no significant differences in photosynthesis or leaf properties in Abutilon grown at 27 Pa CO₂ compared with 35 Pa CO₂, suggesting that the rise in CO₂ since the beginning of the industrial age has had little effect on the photosynthetic performance of Abutilon. For Amaranthus, limitations of photosynthesis were balanced between stomatal and biochemical factors such that net photosynthesis was similar in all CO₂ treatments. Differences in photosynthetic response to growth over a wide range of CO₂ partial pressures suggest changes in the relative performance of C₃ and C₄ annuals as atmospheric CO₂ has fluctuated over geologic time.     

Evolution of C4 phosphoenolpyruvate carboxylase in the genus Alternanthera: gene families and the enzymatic characteristics of the C4 isozyme and its orthologues in C3 and C3/C4 Alternantheras

Phosphoenolpyruvate carboxylase (PEPCase, EC 4.1.1.3) is a key enzyme of C₄ photosynthesis. It has evolved from ancestral non-photosynthetic (C₃) isoforms and thereby changed its kinetic and regulatory properties. We are interested in understanding the molecular changes, as the C₄ PEPCases were adapted to their new function in C₄ photosynthesis and have therefore analysed the PEPCase genes of various Alternanthera species. We isolated PEPCase cDNAs from the C₄ plant Alternanthera pungens H.B.K., the C₃/C₄ intermediate plant A. tenella Colla, and the C₃ plant A. sessilis (L.) R.Br. and investigated the kinetic properties of the corresponding recombinant PEPCase proteins and their phylogenetic relationships. The three PEPCases are most likely derived from orthologous gene classes named ppcA. The affinity constant for the substrate phosphoenolpyruvate (K _0.5 PEP) and the degree of activation by glucose-6-phosphate classified the enzyme from A. pungens (C₄) as a C₄ PEPCase isoform. In contrast, both the PEPCases from A. sessilis (C₃) and A. tenella (C₃/C₄) were found to be typical C₃ PEPCase isozymes. The C₄ characteristics of the PEPCase of A. pungens were accompanied by the presence of the C₄-invariant serine residue at position 775 reinforcing that a serine at this position is essential for being a C₄ PEPCase (Svensson et al. 2003). Genomic Southern blot experiments and sequence analysis of the 3′ untranslated regions of these genes indicated the existence of PEPCase multigene family in all three plants which can be grouped into three classes named ppcA, ppcB and ppcC.     

Interactive effects of atmospheric CO2 enrichment,salinity and flooding on growth of C3 (Elymus athericus) and C4 (Spartina anglica) salt marsh species

The growth response of Dutch salt marsh species (C₃ and C₄) to atmospheric CO₂ enrichment was investigated. Tillers of the C₃ speciesElymus athericus were grown in combinations of 380 and 720 11^-1 CO₂ and low (O) and high (300 mM NaCl) soil salinity. CO₂ enrichment increased dry matter production and leaf area development while both parameters were reduced at high salinity. The relative growth response to CO₂ enrichment was higher under saline conditions. Growth increase at elevated CO₂ was higher after 34 than 71 days. A lower response to CO₂ enrichment after 71 days was associated with a decreased specific leaf area (SLA). In two other experiments the effect of CO₂ (380 and 720 11^-1) on growth of the C₄ speciesSpartina anglica was studied. In the first experiment total plant dry weight was reduced by 20% at elevated CO₂. SLA also decreased at high CO₂. The effect of elevated CO₂ was also studied in combination with soil salinity (50 and 400 mM NaCl) and flooding. Again plant weight was reduced (10%) at elevated CO₂, except under the combined treatment high salinity/non-flooded. But these effects were not significant. High salinity reduced total plant weight while flooding had no effect. Causes of the salinity-dependent effect of CO₂ enrichment on growth and consequences of elevated CO₂ for competition between C₃ and C₄ species are discussed.